For image processing and color graphic applications which require an accurate hardcopy reproduction, photographic media represents the best alternative because of its high resolution, wide dynamic range and color sensitivity.
Several methods may be used to obtain a photographic hardcopy. One method is to use a 35 mm camera on a tripod to take pictures directly off the display monitor. Such a method has several disadvantages: first, geometric distortion would result from the curvature of the screen being photographed; second, imperfect registration of the red, green and blue beams hitting the cathode ray tube phosphor results in loss of contrast and sharpness; also, lack of compensation for the non-linear aspects of film saturation relative to exposure time results in shifts in color and gray levels from the original image. For these and other reasons, a film recorder is required if consistent and artifact free reproductions are to be created.
An example of a film recording system of general applicability for the conversion of video data to photographic hardcopy is provided by a system designed by Focus Graphics Corporation of Redwood City, Calif. These systems operate by sequentially generating three different images, one for each color on the photographic medium, on a precision monochrome CRT. For each color image a different color filter matched to the corresponding film color is interposed between the CRT and the film. The image is recorded under control of the microprocessor, which determines the exposure time and light intensity variations during each image scan. These systems will hereafter be referred to as Computer Image Recorders or CIR.
Because of the rapid development pace in the computer graphics and image processing field, new video generating systems are constantly being introduced which are capable of higher and higher resolutions. Different video resolutions translate into different timing signal characteristics. The horizontal sync frequency in particular will change as the video resolution changes. One very desirable feature for a raster scan system is the ability to be compatible with a broad range of horizontal sync frequencies, and therefore to be compatible with a wide range of video sources. This capability is generally known and will hereafter be referred as multiscan capability.
Various workers in the art have resolved this problem with various degrees of success. A number of systems are known for generating color and pseudo-color displays from input video signals presented in raster scan format. Some of these systems such as the NEC multi-sync monitor are capable of handling a wide variety of video signals, in particular a broad range of incoming horizontal frequencies.
A number of factors have precluded the development of multiscan systems able to produce an accurate conversion to photographic records. It need only be noted that width and height of the image will vary with the horizontal and vertical scan frequencies, affecting the size of the photographic record.
Automatic conversion of the information contained in video signals to a photographic image also requires compensation for changes in horizontal and vertical linearity and should desirably be able to convert video signals with different sync polarities or signals including equalizing and serration pulses.
The techniques used heretofore to handle video signals with different timing signals have relied on phase-lock loop circuits to attempt to achieve multiscan capability. While such techniques can be utilized to convert video signals within a narrow range of horizontal frequencies, they have proved inadequate to satisfy current needs. As a consequence, manufacturers of CIR systems offer a family of systems, each optimized for a certain range of timing signals. At most variations of 5 to 10% of horizontal frequencies have been introduced using conventional phase-lock techniques. In addition, even small variations of horizontal and vertical frequency require lengthy and complicated tuning procedures as width, height, horizontal and vertical linearity of the picture will be affected.
More recently manufacturers of video display terminals have introduced multiscan displays able of covering a two to one range in frequencies by breaking down the overall range into three smaller ranges and electronically selecting the range depending on the video signal. Within each smaller range, the unit is then synchronized using traditional phase-locked loop techniques. Such a technique will exhibit focal points in scan frequencies at which the unit synchronizes satisfactorily. Outside of these focal points the units will generally require some manual adjustments in order to provide adequate synchronization. As a consequence such displays fail to provide consistent width and horizontal linearity making them unsuitable for photographic applications.
Another limitation of existing raster scan systems is their limited vertical sync range. While some will synchronize within a relatively broad range, they will fail to maintain consistent vertical linearity.
Prior art approaches can therefore be seen to be unsuitable for handling the full scope of horizontal and vertical sync frequencies current computer systems can generate in a manner suitable for photographic reproduction, without significant manual adjustments. They also have limited capability for handling video signals with varying video levels, sync polarities and other types of video characteristics.